Cascade electrocatalysis via AgCu single-atom alloy and Ag nanoparticles in CO2 electroreduction toward multicarbon products

Electrocatalytic CO2 reduction into value-added multicarbon products offers a means to close the anthropogenic carbon cycle using renewable electricity. However, the unsatisfactory catalytic selectivity for multicarbon products severely hinders the practical application of this technology. In this paper, we report a cascade AgCu single-atom and nanoparticle electrocatalyst, in which Ag nanoparticles produce CO and AgCu single-atom alloys promote C-C coupling kinetics. As a result, a Faradaic efficiency (FE) of 94 ± 4% toward multicarbon products is achieved with the as-prepared AgCu single-atom and nanoparticle catalyst under ~720 mA cm−2 working current density at −0.65 V in a flow cell with alkaline electrolyte. Density functional theory calculations further demonstrate that the high multicarbon product selectivity results from cooperation between AgCu single-atom alloys and Ag nanoparticles, wherein the Ag single-atom doping of Cu nanoparticles increases the adsorption energy of *CO on Cu sites due to the asymmetric bonding of the Cu atom to the adjacent Ag atom with a compressive strain.

low temperature to remove the oxides and keep the Cu-Ag distribution, then collect the XAS spectra, to exclude the interference from oxide.
7. In the EXAFS fitting of Cu in AgCu SANP, the bond length of first Cu-O shell was 1.952 A, longer than the Cu-O bond in CuO (1.947 A) and Cu2O (1.849).What is the possible reason?The existence of Cu2O was not considered in the fitting.
8. Line 125-126, " As presented in Figure 3E, AgCu SAA presents a much higher FE than that of Cu NP, strongly indicating the improvement of C-C coupling kinetics by Ag single-atom in Cu lattice."The FE of Cu NP is about 87%, and that of AgCu SAA is around 100%.This only indicate HER was suppressed on AgCu SAA.To show the kinetics on two catalysts, you should compare the reaction rate, namely partial current density, rather than FE of C2+ products.Similarly, plots of partial current density should be provided for CO2 reduction.Since the total current densities on Cu and AgCu ASNP are similar (Figure 3C), the enhancement factor of partial current density of C2+ on AgCu ASNP seems less than 2 compared with Cu.The difference between AgCu SAA and AgCu ASNP is even smaller.The enhancement factor less than 2 is not very significant in kinetics.The increasing of FE of C2+ is more related to the suppression of formation of other minor products.9. Following last comment, to compare the intrinsic kinetics of C2+ formation on different catalysts, maybe its better to conduct the CO2 reduction tests in H cell with low loading of catalyst, to guarantee all catalyst particles are accessible for CO2 in solution.The current density needs to be kept low to avoid the rate being limited by mass transport.Then by comparing the ECSA normalized partial current density of C2+ products, the intrinsic activity of different catalysts can be compared.Under this situation, the FE of C2+ may be much lower than the value in flow cell, but more kinetic insights can be obtained.13.Line 174-175: "The main difference between the selectivity of Cu(100) and Ag-doped Cu(100) thus comes from the fact that Ag is deemed to produce CO from CO2." Does "Ag" here mean Ag atom or Ag particle?Is Ag single-atom in Cu or Ag NP the catalyst for CO2-to-CO?If Ag single-atom can play this role, then why AgCu SANP showed higher selectivity to C2+ than AgCu SAA in CO2 reduction.Maybe, the authors should simulate the energy diagram of CO2-to-CO on Ag single-atom and Ag metal to show which one is more active for this step and whether Ag NPs are necessary for the tandem reaction.
14. Line 211: The Chemical section did not provide the information of Cu NPs and AgNO3, the most two important chemicals used in this work.
Reviewer #2 (Remarks to the Author): The author designed an AgCu single-atom alloy and Ag nanoparticle cascade catalyst for CO2 to C2+ products conversion.The catalyst demonstrates 94% total C2+ products selectivity under 720 mA cm-2 current density.The performance ranks at the top level of all the published results.Moreover, the author used a simple and reproducible synthesis method.In the meanwhile, the characterization is comprehensive and detailed.Overall, I support the publication of this manuscript if the following concerns are addressed.
When the AgCu SANP significantly improves the C2+ FE compared to Cu NP and AgCu SAA, the current density remains in a similar range.If the Ag increases the local CO concentration and expedites the C-C coupling, the improvement in current density should be more obvious.Could the author explain more about that?
Could the author explain why there is a 20% missing for the total FE of Cu NP in Figure 3B?It is not a fair comparison to say AgCu SAA improves C2+ FE if the Cu NP result has a large error.Similarly, the AgCu NP data in Figure 3B and AgCu SANP data in Figure 3E all miss a significant part of the total FE. Figure 3F uses "second" as the unit for the time scale, which is not reading-friendly.The author should use "hour" as a unit.
The author claims that ethanol formation is favored over ethylene.Then, why do Cu NP and AgCu SAA generate more ethylene than ethanol during CO2RR?
Reviewer #3 (Remarks to the Author): In this paper, the authors disclosed the synergy effect between AgCu SAA and Ag nano particles for efficient CO2RR.Although a high FE of 94% C2+ products was achieved on their AgCu SANP, no enough evidence could support their results and mechanism.
1.Although a special structure was claimed, the traditional tandem reaction mechanism was still used to explain their results.How is the meaning to constructure this new structure?
2. With the increase of the Ag, there is no doubt to constructure Ag nanoparticles.How about the proportion of Ag SA and nanoparticles?What is the main contribution to the reaction?How to determine Ag SA is close to the Ag nanoparticles?How many Ag SA will improve the C-C coupling and what is the case of nanoparticles close to each other?What is the effect of Ag SA far from the Ag nanoparticles?
3. Usually, Ag shows weak adsorption to *CO.Why the authors considered it will increase the adsorption of *CO for Cu?It is wired that the adsorption of *CO could be enhanced by a weak adsorption one.
4. From the CO2RR, the authors just take the very beginning 100s or less data.In the early stage of the reaction, there are always some fake data.Thus, data from over 20 min reaction is recommended to show its performance.
5. In the CO reduction, the AgCu SANP showed much lower ethanol generation than that of AgCu SAA and Cu NPs.While, it showed much higher ethanol generation than those of AgCu SAA and Cu NPs in CO2RR.This is opposite for their explanation.The same problems are similar for ethylene and acetic acid.
6.The DFT simulation is too simple.
7. There many typing error, such as Figure S7, S8 were wrong in the experimental part.

Point-to-Point Response
Reviewer #1 (Remarks to the Author): This work reported AgCu single-atom alloy catalysts prepared by the galvanic replacement reaction between commercial Cu NPs and AgNO3.The catalyst showed outstanding Faradaic efficiency of C2+ products (94%) and high current density (720 mA cm-2) in CO2 reduction reaction.The performance of this catalyst is very attractive and may qualify this work to be published on Nature Communication.
However, the characterization data of the catalysts were poorly analyzed and the DFT simulation result is not very convictive.Therefore, I recommend this work not to be accepted in the present form.The following comments need to be addressed.
1.In the XRD patterns, as the galvanic replacement proceeded, why the diffractions of CuO became lower while the diffractions of Cu2O became stronger?

Response:
The synthesis of AgCu catalyst is mentioned in the methods section in page 9 of the manuscript: "20 mg Cu NPs and 2 mg AgNO3 were firstly dispersed into 5mL ethylene glycol and 0.2mL H2O, respectively.Then, the two solutions were combined being together and put in an ultrasonic bath for the galvanic reaction for 30 min."The ethylene glycol solvent is reductive, and ultrasonic bath unavoidably increased the temperature of solvent.So, the ethylene glycol can partly reduce the CuO in the Cu NPs into Cu2O.That's the reason why diffractions of CuO became lower while the diffractions of Cu2O became stronger.We add this explanation in the method section in page 9 of the manuscript.
It states "The ethylene glycol solven is reductive, and the ultrasonic bath unavoidably increased the temperature of solvent.Thus, the ethylene glycol can partly reduce the CuO in the Cu NPs into Cu2O (Figure 2a)." To better understand the synthesis method, we want to explain a little more about the method design.
The AgCu catalysts were prepared through the galvanic replacement reaction between commercial Cu NPs and Ag + , which was spontaneously driven by their reduction potential difference.(Advanced Materials, 2013, 25(44): 6313-6333.)However, this replacement reaction is very rapidly due to the relative strong oxidation of Ag + , and easy to form atoms aggregation rather than single-atom catalysts.
Hence, we utilize reductive ethylene glycol solvent to slow down the replacement reaction between Ag + and Cu to control the resulted catalysts.Small amount of water (0.2mL) was added to dissolve the AgNO3 salt since it is hard to dissolve in ethylene glycol.Ultrasonic bath was used to increase the dispersibility of Cu NPs in the reaction solvent, but the solvent temperature increase was unavoidable.
Therefore, the CuO in the Cu NPs were partly reduced into Cu2O by the ethylene glycol under high temperature.By the way, the Cu oxidized by Ag + was in the form of Cu 2+ rather than Cu oxidation.

Response:
Here, the polycrystalline is compared to the single-crystal Cu samples, which has only one lattice orientation in the sample, such as the sample in Scientific reports, 2014, 4(1): 1-6.To avoid possible confusion, we delete this sentence.However, at the atomic scale, the complex electron scattering between the number of X rays detected and number of atoms the probe interacts with, making it impossible to directly relate x ray counts to the number or density of atoms (Microscopy andMicroanalysis, 2016, 22:1432;Microscopy and Microanalysis, 2017, 23(3): 513-517).We can only use the low mag EDX to examine the existence of this element and overall redistribution in figure S3.But it cannot be used at atomic scale to interpret.

XRD patterns indicate the existence of Cu2O, but Cu+ is not considered in the XPS fitting in
There is no Ag signal detected in this localized region in Figure S2G, even in an enlarged EDX spectrum.
It is just below the detection limit at local region.It does not mean Ag atom does not exist in the local region.We added a few sentences at the page 3 of the manuscript.It states "However, at the atomic scale, the complex electron scattering between the number of X-rays detected and number of atoms the electron probe interacts with, make it impossible to directly relate X-ray counts to the number of density of atoms. 22,23Thus, we cannot confirm the existence of Ag single atoms by point spectrum (Figure 2G).
In order to confirm the existence of Ag single atoms, atomic scale imaging is required."

Response:
Thanks for the reviewer's comments on the EXAFS fitting results.EXAFS measures the average structure of every possible structural phase in the material.Based on the Ag single atom sites, Ag would bond directly with Cu; so those single atomic Ag signals would only average with Ag nanoparticles signal, which is more critical to represent the proportion of Ag single atom and Ag nanoparticles.No other Ag-containing structure form involved.However, the Cu structure is much more complex than Ag, which contains CuO, Cu2O, and Cu atoms in the Cu cluster directly bonded to Ag, and the Cu atom in the Cu cluster indirectly bonding to Ag.Based on the Ag fitting and DFT modeling, only a small amount of Cu bonding with Ag in the Cu cluster.Those complex mixtures of Cu phases and a small amount of Cu-Ag bonding make the ratio estimation from Cu EXAFS is not less certain or reliable.As shown in the following response figures 2-3 and response table 1, we could even fit the Cu EXAFS without any contribution from Ag-Cu, but the lack of Ag-Cu contribution in the Ag EXAFS cannot reproduce the spectrum well.In our manuscript, therefore, we added an Ag-Cu scattering path in Cu EXAFS to make consistence and demonstrate that we could also see it from Cu K-edge.To make clearer the interpretation of the coordination number, we add the following comments in the EXAFS fitting table S1 in the supporting information: "Due to complex mixture phases of Cu structures, Cu2O, CuO,

Response:
Thanks for pointing out the inaccurate argument here.The positive shift of the adsorption edge of AgCu SANP in the Cu K-edge may be also caused by the existence of Cu2O and CuO in our system.However, the edge shift of Ag XANES and no Ag-O scattering path existing in Ag EXAFS confirms that Ag and Cu would form a bonding to allow the exchange of some electrons between them.We revised the statement in page 4 of our manuscript.We also added the standard spectra of CuO and Cu2O in figure S5 in the supporting information for comparison.

Response:
For the theoretical model CuO and Cu2O we used for EXAFS fitting, the Cu-O in CuO is about 1.956A and 1.848 in Cu2O.Our fitting bonding length is about 1.952A.Considering the error bar, the value falls between two values.For EXAFS fitting, it is a local structure instead of the entire bulk structure or crystal structure for XRD.We only need Cu-O scattering path either from Cu2O or CuO, and the simulation process would adjust the scattering path length (bonding length) to fit the real experimental data.In addition, the real system would be different from the idea model which may have some cell expansion or distortion, particularly in the nanoparticle or nanocluster system.It could also increase the bonding length to some extents.

Response:
Thanks for pointing out this and sorry for our misunderstanding on the kinetics.We agree with you that the increasing of FE of C2+ is more related to the C-C coupling selectivity rather than the kinetics.
Actually, the main novelty of this work is the high selectivity of C2 products, which is agree well with the C-C coupling selectivity improvement.We have corrected the statements based on your comments and also provide partial current density in new figure S11.This suggestion is very helpful.
We revised the statement in page 5. To further demonstrate our understanding, we conducted the H-cell test using 0.1M KHCO3 electrolyte using glassy carbon electrode with low loading of catalyst.As shown in below images, the CO2 reduction activity in neutral electrolytes is relatively low.In particular, the highest total CO2 reduction FE are lower than 30% with a highest C2+ FE of ~23% in AgCu SANP.Especially, the major C2+ product over AgCu SANP is acetate, which is much different to that in KOH electrolyte.
10. Line 152-153, "To model AgCu SAA, we adapt an Ag-doped Cu model structure."What is the structure of this "Ag-doped Cu model"?Line 167-168, "In pure Cu(100) surface the Cu-Cu bond length is 2.57 Å, however, after doping, this is compressed to 2.50 Å at the surface."Which Cu-Cu bond is compressed?The bond between two Cu next to Ag atom？Please draw the structure of cells used for simulation and indicate the bond mentioned.11.Line 161-163: "Prior studies have reported that on Cu electrodes, the C-C coupling can occur through *CO-*CO dimerization, *CO¬¬-*CHO, or *CO-*COH with *CO-*CO coupling being less feasible with a higher transition state (TS) energy (> 1 eV).31-33"Do the authors mean *CO-*CHO and *CO-*COH couplings are less feasible than *CO-*CO coupling?If so, why the author only simulated *CO-*CHO and *CO-*COH couplings in the following sections.I think more previous works regard *CO-*CO coupling as the predominant pathway for C-C bond formation, including references 31-33.In some reports, *CO-*CO is an electron-transfer step decoupled with proton transfer.Decoupled electron-proton transfer is more difficult to treat in DFT simulation.Is this part of the reason the authors chose to simulate *CO¬¬-*CHO and *CO-*COH couplings.12. Figure 4a and 4b only show the energy diagram on Ag-doped Cu (100).The authors should overlay the energy diagram on pure Cu(100) to show how much difference the Ag atoms induced.Line 170-174: "As shown in Figure 4A, the TS values of C-C coupling via *CO-*CHO and *CO-*COH reactions are 0.51 and 1.10 eV, respectively, … A transition state energy of 0.55 eV barrier is obtained, which is slightly higher than the Ag-doped Cu(100) surface."What is the common error for DFT simulated activation energy?Is 40 meV difference of couple barrier a significant difference?Does the doping of Ag significantly facilitate the coupling step?

2.
Line 79-Line 81: "One should note that it contains Cu (111), (200), and (220) reflections in all the samples (Figure S4), which indicates Cu is polycrystalline."What does "polycrystalline" here mean?Does it mean the whole sample was polycrystalline or a Cu nanoparticle is polycrystalline?A powder sample of single-crystalline Cu nanoparticles will also generate diffractions of different facets since the nanoparticles are not oriented through any self-assemble process.

Figure 2C .
Figure 2C.Usually, large satellite features indicate the existence of Cu2+.Cu+ doesn't show strong satellite feature.The binding energies of 2p electron in Cu+ and Cu0 are similar, but Cu+ and Cu0 can be distinguished by Auger spectrum.Therefore, the Auger spectrum of the catalysts should also be analyzed.

4.
Figure S2G: "EDX of region 2, which only shows the signal of Cu without Ag due to the low content of Ag in AgCu SAA composition."If Ag single atoms can not be detected by EDX, then why the EDX mapping in Figure S3E still shows the uniform distribution of Ag?An enlarged EDX spectrum of the AgCu SAA region need to be provided to show whether the signal of Ag is above the noise.Response: EDX STEM can analyse the chemical composition of materials at low magnification.
AgCu SANP, the Ag-Ag and Ag-Cu coordination number are 7 and 1, respectively.Indicating more Ag atoms were in Ag NPs instead of SAA.The Cu-Cu (metal + oxide) and Cu-Ag coordination number are 3.5 and 1, respectively.Considering that the fraction of Ag was only 1 wt% in this sample and only a small fraction of Ag atoms was SAA, the ratio of coordination numbers of Cu-Ag/Cu-Cu as 1:3.5 seems not very reasonable.The coordination number of Cu-Ag seems too high.
Cu atoms bonded to Ag in Cu cluster, and Cu atoms not bonded to Ag in Cu cluster, the coordination number of Cu-Ag extracted from Cu EXAFS spectral is just the demonstration of existence Cu-Ag."Response Figure 2: Fourier Transfer R-space of Cu K-edge EXAFS experimental and fitting spectrum of AgCu SANP without Cu-Ag scattering path Response Figure 3: Fourier Transfer k-space of Cu K-edge EXAFS experimental and fitting spectrum of AgCu SANP without Cu-Ag scattering path

6.
Line 97-98: "The positive shift of the adsorption edge of AgCu SANP in the Cu K-edge also demonstrates the electron loss of Cu (Figure S5)."The XRD has shown the existence of CuO and Cu2O in this sample.The edge positions of CuO and Cu2O are more positive than Cu foil.Therefore, the positive shift of the edge did not indicate the electron transfer from Cu to Ag.The standard spectra of CuO and Cu2O should be added in Figure S5A.If possible, the author should reduce the sample by H2 at relative low temperature to remove the oxides and keep the Cu-Ag distribution, then collect the XAS spectra, to exclude the interference from oxide.
5 of AgCu SANP shows a slight shift to lower energy compared to that of Ag foil, indicating electron transfer from Cu to Ag due to the formation of Ag-Cu bond.Since the co-existence of Cu2O and CuO, it is hard to distinguish either the positive shift of the adsorption edge of AgCu SANP in the Cu K-edge is caused by electron transfer from Cu to Ag or the electron transfer from Cu to O (Figure S5).However, Fourier transform (FT) of the k 2 -weighted extended X-ray absorption fine structure (EXAFS) curve of the Ag K-edge of AgCu SANP shows both Ag-Ag and Ag-Cu coordination bond (Figure 2E), and lack of Ag-O scattering path, which confirms the electron transfer between Cu and Ag." Response Figure 4: New Figure S5a Due to the limited experimental conditions, it's very hard for us to use H2 reducing the samples at synchrotron.We are so sorry on this point and hope you can understand.7.In the EXAFS fitting of Cu in AgCu SANP, the bond length of first Cu-O shell was 1

8.
Line 125-126, "As presented in Figure 3E, AgCu SAA presents a much higher FE than that of Cu NP, strongly indicating the improvement of C-C coupling kinetics by Ag single-atom in Cu lattice."The FE of Cu NP is about 87%, and that of AgCu SAA is around 100%.This only indicate HER was suppressed on AgCu SAA.To show the kinetics on two catalysts, you should compare the reaction rate, namely partial current density, rather than FE of C2+ products.Similarly, plots of partial current density should be provided for CO2 reduction.Since the total current densities on Cu and AgCu SANP are similar (Figure 3C), the enhancement factor of partial current density of C2+ on AgCu ASNP seems less than 2 compared with Cu.The difference between AgCu SAA and AgCu ASNP is even smaller.The enhancement factor less than 2 is not very significant in kinetics.The increasing of FE of C2+ is more related to the suppression of formation of other minor products.
It states "Again, the concept of this proposed AgCu SANP cascade catalysis is that the Ag single-atom on Cu can promote the C-C coupling selectivity, while the Ag nanoparticles can produce local CO from CO2.To further prove this point, CO reduction experiments were carried out to study the C-C coupling selectivity.As presented in Figure 3E, AgCu SAA presents a much higher FE than that of Cu NP, strongly indicating the suppression of HER and improvement of C-C coupling selectivity by Ag single-atom in Cu lattice.""Similarly, Figure S11 shows the C2+ partial current density at -0.65V increased from 353 mA cm -2 (Cu NP), 553 mA cm -2 (AgCu SAA) to 677 mA cm -2 (AgCu SANP), which proves that Ag increases the local CO concentration and expedites the C-C coupling.Response figure S5: Figure S11.(A)The partial current density of C2+ products of Cu-based samples; (B) The FE values of various products during the long-term stability test.Noted that the FE of H2 products from competitive hydrogen evolution reaction is not presented.9. Following last comment, to compare the intrinsic kinetics of C2+ formation on different catalysts, maybe it's better to conduct the CO2 reduction tests in H cell with low loading of catalyst, to guarantee all catalyst particles are accessible for CO2 in solution.The current density needs to be kept low to avoid the rate being limited by mass transport.Then by comparing the ECSA normalized partial current density of C2+ products, the intrinsic activity of different catalysts can be compared.Under this situation, the FE of C2+ may be much lower than the value in flow cell, but more kinetic insights can be obtained.Response:Thanks for your recommendation on the kinetics tests.Our study is conducted in a flow basic electrolyte (1M KOH) in the flow cell.But if we move the experiments in a H-cell, the electrolyte inevitably change into neutral solution, because CO2 will react with KOH and form KHCO3 or K2CO3.That means, there is no possible way to keep the electrolyte in H-cell to be the same with that in flow-cell.If electrolytes change, pH will also be different, and the experiment results in H-cell may be totally different to that in flow-cell.We tried to simulate the flow cell in a H-cell by importing CO2 gas into KOH electrolyte, but it's not stable.Thus, we conducted the test in H-cell with a neutral electrolyte KHCO3 electrolyte.We tried to test in neutral electrolyte (KHCO3), but the activity trends changed a lot.The pH of electrolyte also affects the performance a lot, which agree well with previous work, such as ACS Energy Letters, 2020, 5(10): 3101-3107; ACS Energy Letters, 2018, 3(4): 812-817; etc.It is very hard to study the kinetics of CO2 reduction under alkaline electrolyte.Considering the emphasis of this work is on the selectivity of C2 products, we think the lack of kinetics doesn't affect the main conclusion.
Response Table1Fitted EXAFS parameters at the Cu and Ag K-edge for AgCu SANP without Ag-Cu scattering path.